Mendel Dreams:
The Beginning of Genetics
Clyde Freeman Herreid
Department of Biological Sciences
University at Buffalo
Paula P. Lemons
Division of Biological Sciences
University of Georgia
Learning Objectives
• Describe some of the history of Mendel’s work on
patterns of inheritance.
• Explain the blending hypothesis.
• Explain the law of the segregation of alleles.
• Explain the law of independent assortment.
• Set up a Punnett square correctly.
• Draw conclusions about inheritance from genetic cross
data.
• Predict outcomes from genetic cross experiments.
• Design genetic cross experiments to answer questions
about inheritance and analyze genetic cross experiments
to improve them.
2
3
He was dying.
And it was January.
As was fitting for an old man with swollen
feet, a disagreeable kidney, and an
untenable position on the chessboard, he
closed his eyes.
4
He dozed in an old man’s way, thinking not
of yesterday’s battle with the city alderman
over the infernal tax problems of the Abbey,
but of his childhood days on the farm, his
sister who had forfeited her dowry so he
could go to school, and his parents.
He pulled his cleric’s robe tighter around his
too ample frame. He was cold. He waited.
For Leos. The choirmaster had promised he
would play at his funeral. He smiled and
remembered him as a boy.
5
For sixteen years he had
been the Abbot of the
Abbey—a great honor.
But it had drained his
strength; he could no
longer tend to his
scientific studies.
Still there were compensations. Yet at this
moment, on this cold January 6th, he had
difficulty remembering what they were.
6
But mostly he remembered his beloved
garden in the abbey where he worked so
long ago… eight years with 29,000 pea
plants to tend.
7
How long ago was it? Twenty? Thirty years?
“Here boys, listen.” He was once again a
teacher at Znaim.
“Listen, boys. It is possible to cross breed
plants just like animals. One has to take the
pollen from one plant with a small brush and
carefully, so carefully, place it on another.”
8
“Here I take the pollen from the male and
place it upon the carpel of another plant.
Then the plant does the rest.”
9
“But why would you do that, Father?”
“Because I want to know the rules. There
must be rules—laws of breeding, just as
there are laws of physics.
“I want to find those rules. That is why,
Master Pavel, I seek the answers.”
10
“But how do you stop a bee bringing pollen
from one plant to another to mix up your
results, Father?”
“Try to guess. What should I do?”
“Possibly bring the plant inside, or cut the
anther off before you pollinate the plant or
cover the plant with a little sac? Or . . .
maybe, Father, you just pray?”
11
“Yes, I did most of those things.
“Sometimes I did work in the greenhouse.
When I worked in my garden, I covered
the flower with a little cheesecloth bag.
And I did have to remove the anther of
the flower so that it would not fertilize
itself.
“And most assuredly I did pray.”
12
“I was particularly interested in testing the old idea
that the traits of the offspring were a simple blending
of the characteristics of the two parents. You boys
all know that you look like a combination of your two
parents.
“Pavel, you have your mother’s nose and sense of
humor and your father’s eyes and his musical talent.
“The same with all of us.
“But I had long wondered what would happen if I
looked carefully at individual traits. What then?”
13
Clicker Question 1
“Here is one of my experiments. What do you think would
happen when I breed a white-flowered plant with a purpleflowered plant, if the blending hypothesis is correct?”
A. All of the flowers in the next generation would be white.
B. All of the flowers in the next generation would be purple.
C. All of the flowers in the next generation would be light
purple.
D. Some of the flowers in the next generation would be
white and some purple.
E. The next generation would not flower at all.
14
“Here is what did happen. It is not at all what
the plant scholars anticipated.”
15
“How can that be, Father? Where did the white go?
Purple must be stronger than white. I say it
dominates the white.”
“I think so too, Pavel. I think so too,but not in the way
you imagine.
“Let me show you another cross, this time between a
tall plant and a short plant.
Notice, I am only watching one trait at a time. I could
be watching the flower color too, but that would be
too complicated. Let’s just look at the height of the
plant.”
16
Monohybrid Crosses
(Breeding experiments involving one trait)
P
Tall x Tall
Short x Short
F1
All Tall
All Short
“Here I breed two tall parents with each other and
they breed true. They produce all tall children.
“And I breed two short plants and they breed true,
yielding all short.”
17
“But look what happens when I breed a tall
and short together.”
P
Tall x Tall
Short x Short
F1
All Tall
All Short
F2
All Tall
18
“It is the same thing, Father. The short has
disappeared. Where has it gone? Tall is
stronger than short.”
P
F1
F2
Tall x Tall
Short x Short
All Tall
All Short
All Tall
The tall trait is dominant
19
“The same thing happened with these traits
too. The ones on top are dominant over
those below. What could I conclude?”
20
Clicker Question 2
Which of the following conclusions could Mendel draw from
the breeding experiments we’ve examined to this point? There
may be more than one good answer to this question. Choose
the one you think is best, and be prepared to discuss each
answer.
A. Parents’ traits blend together in their offspring.
B. Parents’ traits do not blend together in their offspring.
C. It is not possible to predict what version of the trait will be
present in the offspring.
D. One version of the trait is lost during reproduction.
E. Each of the different versions of that trait maintain a discrete
identity in the offspring.
21
“I conclude that the traits
do not blend together in
the offspring of two
parents. The blending
hypothesis can be
rejected!”
The reason that everyone
thought blending was right was
because they never looked at
individual traits.
22
“Now look what happens when I breed the two F2
generation tall plants together.”
F1
All Tall
F2
Tall
All Short
Short
Tall
787 Tall
x
and
Tall
297 Short
23
Clicker Question 3
“With these new data, what should I conclude?” Note that there
may be more than one good answer to this question. Choose
the one you think is best, and be prepared to discuss each
answer.
A. This is additional evidence that traits do not blend together in
the offspring of two parents.
B. It is not possible to predict what version of the trait will be
present in the offspring.
C. One version of the trait is lost during reproduction.
D. Each of the different versions of that trait maintain a discrete
identity in the offspring.
24
“I concluded that the
factors that control a trait
maintain a discrete
identity when passed
from parent to offspring.”
This became known as my
Law of Segregation
25
Clicker Question 4
In humans the allele for glaucoma is dominant over the allele for
normal eyes. Suppose a man with glaucoma marries a woman
with normal eyes and they have children. If this trait is inherited
like height in pea plants, which of the following would be true?
A. All of their children will have glaucoma, but only some of their
grandchildren will.
B. None of their children will have glaucoma, and some of their
grandchildren will.
C. Some of their children may have glaucoma, and if the children
with glaucoma reproduce with someone who also has glaucoma,
most of those people will have glaucoma but some will have
normal eyes.
D. There is insufficient data to determine which, if any, of these
answers is true.
26
Back to Mendel. . .
“But I noticed something strange. The dominant traits were
always more common than the recessive traits…”
Trait
Dominant vs. Recessive
Phenotypes
Dominant vs. Recessive
Frequency
Seed shape
Round vs. Wrinkled
5474 vs. 1850
Seed color
Yellow vs. Green
6022 vs. 2001
Pod shape
Full vs. Constricted
882 vs. 299
Pod color
Green vs. Yellow
428 vs. 152
Flower color
Purple vs. White
705 vs. 224
Flower and pod position
Axial vs. Terminal
651 vs. 207
Stem length
Tall vs. Short
787 vs. 277
27
“… and they were in almost the same ratio.”
Dominant vs. Recessive
Phenotypes
Dominant vs. Recessive
Round vs. wrinkled seed shape
5474 / 1850 = 2.96
Yellow vs. green seed
6022 / 2001 = 3.01
Full vs. constricted pod
882 / 299 = 2.95
Green vs. yellow pod
428 / 152 = 2.82
Purple vs. white flower
705 / 224 = 3.15
Axial vs. terminal position
651 / 207 = 3.14
Tall vs. short length
787 / 277 = 2.84
75% show dominant traits
25% show recessive traits
28
“How could you explain this, Pater Gregor?”
“My mathematical training in the university
came to my aid. My physics professor, Herr
Doppler, always encouraged us to think
mathematically. I asked myself could I explain
this 3:1 ratio simply?”
“Yes. Yes, and again, yes.”
29
“Suppose that the traits were factors like particles.
“We could list them this way:
the dominant factor T = tall plant
the recessive factor t = short plant.
“Imagine that a pure bred tall
plant would have 2 TT & a
pure bred short plant would
have 2 tt.”
30
“Then when I bred them TT x tt = ?
“If they would only pass on one of their
traits via the pollen or the egg, then what?”
“I have it, Father. I have it!
The answer would be Tt.”
31
“But, Father, that would mean that the tall hybrids
would be carrying a factor for shortness.”
“Exactly, exactly. But you cannot see the shortness.
It is hidden. And what would happen when two
hybrids were bred? Tt x Tt = ?”
“I know, Father, there would be 1 pure tall plant, 2
hybrid plants, and 1 short plant.”
“Exactly!”
32
MENDEL ANALYZES THE DATA
Traits passed in the gametes
Pollen (sperm) & eggs
• One factor (allele) comes from the pollen
• One factor (allele) comes from the egg.
Note: ¾ Tall= TT, Tt, tT
¼ Short= tt
3:1 Phenotypic Ratio
33
Possible gamete combinations
Tt
x
Tt = tT, Tt,TT
tt
TT & Tt
T
Notice the 3:1 ratio
34
Clicker Question 5
Look at the glaucoma question again. What information
would be most helpful to you in predicting with more
certainty whether the offspring of these parents will have
glaucoma or normal eyes: a man with glaucoma (dominant)
marries a woman with normal eyes (recessive) and they
have children.
A. Whether the man’s parents had glaucoma.
B. Whether the woman’s parents had glaucoma.
C. Whether the man is “pure” for glaucoma.
D. Whether the woman is “pure” for normal eyes.
35
Using a Punnett Square
Alleles= Alternative forms of the same gene (T or t)
Homozygous
dominant
T
Eggs
t
Suppose Tt X Tt
Heterozygous
Pollen
T
TT
Tt
t
tT
tt
Heterozygous
Homozygous
recessive
36
Terms and Principles
•
•
•
•
•
•
•
Dominant (e.g., Tall)
Recessive (e.g., Short)
Monohybrid crosses involve following 1 trait
Dihybrid crosses involve following 2 traits
Phenotype (visible characteristics)
Genotype (genetic characteristics)
Particulate theory of inheritance = 2 particles
(factors) determine a trait
37
Terms and Principles
•
•
•
•
•
Alleles
Homozygous dominant
Homozygous recessive
Heterozygous
Punnett square
38
Clicker Question 6
“Now students, suppose I wanted to test my
particulate hypothesis of inheritance. Let’s say you
gave me a tall plant, and asked me to tell you what
factors [alleles] it carried? How might I do it?”
A. Breed two unknown plants together.
B. Breed the unknown with any tall plant.
C. Breed the unknown with any short plant.
D. Breed the unknown with a tall heterozygous plant.
E. Breed the unknown with a short heterozygous plant.
39
Answer Slide
“Here is what I did:
“If the unknown plant is tall,
it must be either TT or Tt.
“If I breed it with a pure short plant, tt, what
are the predicted outcomes?”
40
Clicker Question 7
What are the predicted outcomes from this experiment?
A. Either all tall plants or approximately 75% tall plants and
25% short plants.
B. Only 75% tall and 25% short.
C. Only tall plants.
D. Either all tall plants or approximately 50% tall plants and
50% short plants.
E. Only 50% tall and 50% short.
41
“Here is what I did:
“If the unknown plant is tall,
it must be either TT or Tt.
“If I breed it with a pure short plant, tt, then…”
Here are the possibilities:
TT x tt
or
Tt x tt
42
Clicker Question 8
“Now Leos, suppose the results were 57 tall and
43 short, what would you conclude?”
A. The unknown was a pure tall.
B. The unknown was heterozygous for height.
C. There was an error with the experiment because these
results are nothing like I predicted.
43
Gregor Mendel opened his eyes and saw
Leos quietly putting coal on the fire.
The monk reached over to his cold cigar.
Holding it to a candle, he lit it, puffing
deeply. He knew it was bad for his heart—
his kidneys—his everything.
A disgusting habit, he thought.
He reached for his old notes, and paged
along.
44
Gregor Mendel knew he was dying. He had
had a good life. He communicated with
scientists and worshiped with God.
Even now in his latter years when the
business of the Abbey was so irritating, he
still had his music and chess and his cigars.
Always his twenty cigars a day.
A small vice that God would forgive.
45
He no longer could breed mice, for they
escaped too often and stunk too much. But
he had his honey bees and his
meteorological reports—his records. He
always kept meticulous records.
Records were essential, otherwise his pea
experiments would have been impossible.
How else could he have been able to keep
track of two traits or more when he was
breeding?
46
He found the section in his notes, called
Dihybrid Crosses, where he analyzed 2
traits at the same time.
Given that he wanted to breed a pure round
yellow seed plant (both were dominant
traits) with a pure wrinkled green seed (both
were recessive traits), he decided that he
would write it this way:
RRYY
x rryy
47
Dihybrid Crosses
(Breeding experiments involving two traits)
Seed shape = Round (R) or wrinkled (r) seeds
Seed color = Yellow (Y) or green (y) seeds
P
F1
RRYY
x
RrYy
Round Yellow
rryy
Genotype
Phenotype
48
Record keeping was essential. Mendel thought the
traits for seed shape and color were inherited
completely separate from one another. Suppose he
was correct. Set up a Punnett square for RrYy x RrYy
Fill in the headings
for the squares
Fill in
49
Clicker Question 9
RR
Suppose someone set the
Punnett square up this
way for the following cross:
RrYy x RrYy . What would
you say?
rr
YY
yy
RR
rr
YY
yy
A. Correct because the eggs and the sperm are written the same way.
B. Correct because the gametes carry two alleles for each trait.
C. Correct but you should understand that there are many ways to set
up a Punnett square for RRYY x rryy.
D. Not correct because the gametes should carry only one allele for
each trait and each gamete should have an allele for both traits.
50
Dihybrid Cross
RrYy x RrYy
RY
Ry
rY
ry
RY
RRYY RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Find the combinations where there are round yellow seeds.
51
How many round yellow seeds will there
be out of 16 possibilities?
RY
Ry
rY
ry
RY
RRYY RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
• 9 out of 16
52
Possibilities
RrYy x RrYy
RY
Ry
rY
ry
RY
RRYY RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
9:
Round
Yellow
3:
Round
Green
3:
Wrinkled
Yellow
1:
Wrinkled
Green
53
RATIO
9:
3:
3:
1
Round
Yellow
Round
Green
Wrinkled
Yellow
Predicted
56
19
19
6
Actual
55
19
17
4
Wrinkled
Green
• Consistent with the predicted 9:3:3:1 ratio.
54
Mendel’s Data for the Dihybrid Cross
• Consistent with the predicted 9:3:3:1 ratio.
• Mendel called this condition independent
assortment
– Factors (genes) sort independently of one
another when passed from parent to offspring.
55
Clicker Question 10
In humans, a gene controls formation of muscles in the tongue that allow
people with those muscles to roll their tongues, while people who lack those
muscles cannot roll their tongues. The ability to roll one’s tongue is dominant
over non-rolling.
The ability to taste certain substances is also genetically controlled. For
example, there is a substance called phenylthiocarbamate (PTC for short),
which some people can taste (the dominant trait), while others cannot (the
recessive trait).
Suppose a woman who is both a homozygous tongue-roller and a non-PTCtaster marries a man who is heterozygous for tongue-rolling and PTC tasting.
Let R represent tongue-rolling, r represent a non-roller, T represent ability to
taste PTC, and t represent non-tasting.
What is the predicted phenotypic ratio of tongue-rolling and PTC-tasting
among their children?
56
Clicker Question 10
What is the predicted phenotypic ratio of tonguerolling and PTC-tasting among their children?
A. 9 rolling, tasting: 3 rolling, non-tasting: 3 non-rolling,
tasting: 1 non-rolling, non-tasting.
B. 1 rolling, tasting: 1 rolling, non-tasting.
C. 1 rolling, tasting: 1 rolling, non-tasting: 1 non-rolling,
tasting: 1 non-rolling, non-tasting.
D. 3 rolling, tasting: 1 rolling, non-tasting
E. All rolling, tasting.
57
Gregor Mendel died on January 6, 1884, of
chronic nephritis. He entered the Abbey at the age
of 21 and was ordained at the age of 25.
He worked in his garden studying peas from 1856
to 1863 studying 29,000 plants and published his
landmark paper, but no one took serious notice. It
was too mathematical and went against the
prevailing view of blending characteristics.
But he was always fond of saying, “My time will
come.”
58
Mendel is credited with three major ideas:
1)The Particle Theory of Inheritance – which states that
hereditary traits act like particles, units, or factors as they
are passed from generation to generation.
2) His Law of Segregation – which states that his hereditary
factors do not blend but remain distinct during breeding—
thus, disproving the blending theory.
3) His Law of Independent Assortment – which states that
character traits are not connected but are inherited
independent of one another. This law was soon to be
modified.
59
When Mendel died on
that January 6, 1884, at
the age of 61, the great
composer, Leos Janacek,
played the organ at his
funeral as promised.
To his friends’ sorrow his papers were
burned by the abbot who succeeded him.
60
Twenty-five years later his work was
rediscovered by three botanists and his
work became famous. His time had come.
61
Image Credits
Slide 1:
Description: Fresh green peas, Fotolia image #34423680.
Source: http://us.fotolia.com/id/34423680
Clearance: © vadim yerofeyev - Fotolia.com. Licensed royalty free image.
Slide 3, Slide 62:
Description: Group photograph of the Augustinian friars at the Abbey of St Thomas.
Source: Mendelianum, Maravian Museum, Brno
http://www.mendel-museum.com/eng/1online/popup/popup.php3?id=r1_a_1
Slide 4:
Description: Snowy day photo
Source: http://www.publicdomainpictures.net/view-image.php?image=3439&picture=snowy-day
Clearance: Public domain
Slide 6:
Description: Mendel.
Source: http://commons.wikimedia.org/wiki/File:Mendel_Gregor_1822-1884.jpg, from The History of
Biology de Erik Nordenskiöld, Ed. Knopf, 1928.
Clearance: Expired copyright
62
Slide 7:
Description: Augustinian Monastery and Basilica of the Assumption of Our Lady, Brno, Czech
Republic.
Source: http://commons.wikimedia.org/wiki/File:StThomasAbbeyBrno.jpg, by Gareth Simkins (also
known as User:Parmesan) in April 2005, scanned and uploaded in November 2005.
Clearance: Used in accordance with the Creative Commons Attribution-Share Alike 3.0 Unported
license.
Slide 9:
Description: Depiction of artificial pollination.
Source: University of Waikato,
http://www.biotechlearn.org.nz/themes/mendel_and_inheritance/images/cross_pollination_of_pea_pl
ants
Clearance: Used in accordance with educational usage guidelines as described at
http://www.biotechlearn.org.nz/about_this_site/copyright_and_privacy
Slide 20:
Description: Diagram showing the seven "characters" observed by Mendel
Source: Modified from an image by Mariana Ruiz, available at Wikimedia Commons,
http://commons.wikimedia.org/wiki/File:Mendel_seven_characters.svg
Clearance: Public domain.
Slide 60:
Description: Leoš Janáček in 1881.
Source: Cropped version of a photo from
http://commons.wikimedia.org/wiki/File:Janacek_with_wife.jpg, identified as coming "from the
archive of the Department of Music History of the Moravian Regional Museum in Brno.”
Clearance: Expired copyright.
63